1. Field of the Invention
This invention relates to a printed circuit board to be used in electronic equipment, in particular a printed circuit board for which electromagnetic interference (EMI) preventing means is provided.
2. Description of the Prior Art
The recent progress of electronic equipment is facilitating the trend toward increase of operation speed and density of circuits formed as printed circuit board, as a result which regulations are becoming more severe. One of the typical EMI preventing means which are conventionally applied to reduce the radiation noise and incoming noise is covering the printed circuit board with a shield case.
However, this method causes a problem in that the electromagnetic wave energy enclosed in the shield case is emitted outwards through cables. Moreover, this method is not basically intended to reduce the electromagnetic wave energy which is generated due to high frequency characteristics of the circuit. Therefore it is almost impossible to completely suppress the radiation noise. Hence, a new printed circuit board directed to suppress EMI has been proposed (U.S. Pat. No. 4,801,489). This printed circuit board features that an insulation layer (hereinafter this insulation layer is called the undercoat layer) is formed, excepting at least a part of ground pattern, on a substrate on which a signal line pattern (hereinafter referred to as signal pattern), a ground line pattern (herein after referred to as ground pattern) and a power line pattern (hereinafter referred to as power supply pattern) are formed, and a conductive layer such as a conductive paste layer is formed thereon so as to be connected to an uninsulated part of the ground pattern.
Usually, another insulation layer (hereinafter referred to as an overcoat layer) is formed on this conductive layer.
Radiation noise can be reduced in printed circuit boards of such a configuration mainly for the following four reasons.
Firstly, reduction of ground pattern impedance is provided by the conductive layer.
Secondly, removal of high frequency component from the signal pattern and power supply pattern are provided by the adjacent conductive layer. That is, since the undercoat layer which is formed with so-called solder resist is thin, about 20 to 40 .mu.m in thickness, the distributed electrostatic capacity of the signal pattern and power supply pattern with respect to the conductive layer which is formed thereon and connected to the ground pattern is significant. Accordingly, unnecessary high frequency components which are generated by ringing are grounded to the ground pattern with high frequency, so that the radiation noise is suppressed.
Thirdly, equalization of impedance of the signal pattern and power supply pattern are provided by the conductive layer. That is, since the signal pattern and the power supply pattern are covered by the conductive layer, the distance between these circuit patterns and the conductive layer connected to the ground pattern is equalized, and the impedance of each circuit pattern is equalized. As a result, impedance mismatch in high frequency transmission and generation of unnecessary high frequency components which are caused thereby are suppressed.
Another reason is a shielding effect caused by conductive layer itself.
Owing to these four reasons, that is reduction of impedance of the ground pattern by the conductive layer, removal of high frequency components by the adjacent conductive layer, equalization of impedance of the circuit pattern, and the ordinary shielding effect of the conductive layer itself, the radiation noise can be effectively suppressed.
However, in the conventional printed circuit board employing conventional EMI preventing means, the width of the ground pattern lead to the IC ground pattern is small, so that the impedance at high frequency is high, resulting in the occurrence of a potential difference along its length and, thereby, in an unstable ground level of the IC. As a result of fluctuation of this ground level, radiation noise is caused, and by the same reason radiation noise is caused in the output signal of the IC. Due to this, a sufficient effect of the EMI preventing means cannot be obtained.
Usually, in the electronic circuit board, a bypass capacitor to bypass high frequency noise is provided on the connector located at the end of the substrate, power supply terminal and ground terminal. The required function of this bypass capacitor is to bypass the unnecessary high frequency components. Therefore a bypass capacitor of relatively small capacitance is used. In the conventional printed circuit board, this bypass capacitor is mounted on the substrate as a discrete part. Therefore soldering is required to mount the capacitor on the substrate, which impedes reduction of the size of the substrate. Moreover, the lead wire of the bypass capacitor causes inductance at high frequency, as a result of which the frequency characteristics of impedance are degraded.
Generally, in the conventional printed circuit board, a proper number of bypass capacitors are provided between the ground pattern and the circuit pattern or between the power supply pattern and the ground pattern so as to bypass the unnecessary high frequency components. In the conventional printed circuit board, however, this bypass capacitor is mounted on the substrate by soldering as a discrete part. Therefore, soldering is required, and existence of the bypass capacitor impedes the reduction of size of the substrate.
Moreover, the lead wire of the bypass capacitor causes inductance at high frequency, as a result of which the frequency characteristics of impedance are degraded.
In the conventional printed circuit board having the feature that the conductive layer is formed on the undercoat layer, the conductive layer is formed only around the IC excepting the part just below the IC. Therefore the effect of conductive layer could not be sufficiently utilized.
As a result of persistent examination and tests of the above-mentioned printed circuit boards it has been revealed that sufficient effect cannot be obtained as to reduction of radiation noise (high frequency noise) of electromagnetic wave for some shapes of signal pattern included in circuit pattern. There is a problem that if the interior the angle of signal pattern of a bent part is near 90.degree., the radiation noise reduction effect cannot be obtained sufficiently. The reason is unknown yet. It is assumed that the impedance mismatching occurs in the bent part, resulting in generation of a reflected wave, which causes increase of equivalent high frequency impedance. Especially, for the fine pitch high integration circuit board, CAD is used for designing the circuit pattern. Most of circuit boards having the circuit pattern which has been designed with the aid of this support device feature that the bent part of signal pattern has angle of 90.degree.. As the rule of wire width and pitch of signal line of such a board is made more strict, the radiation noise becomes more noticeable.
The layout of a printed circuit board is varied depending on uses. Hence, in some cases sufficient ground pattern area to connect a conductive layer cannot be provided. In these cases the area to connect the ground pattern and the conductive layer is small, and high frequency impedance is increased, resulting in reduction of effect. To prevent potential difference in the ground circuit, it is desirable to connect the ground pattern and the conductive layer in several places. If such a connection is impossible owing to restrictions of layout, the requirement to reduce EMI could not be satisfied sufficiently.